The present invention relates to a method for preparation of olefin oligomers, and particularly relates to a method for preparation of olefin oligomers which can perfectly prevent halogen from getting mixed in the polymerization product and provide olefin oligomer having a low viscosity and a high viscosity index at a high yield.
As the well-known methods for polymerization of olefins, there are a method employing Lewis acid catalyst such as aluminum chloride, a method employing a mixture of aluminum chloride and metallic aluminum powder or metallic zinc powder as the catalyst, etc. There is also known a method of effecting polymerization by employing a liquid catalyst prepared by dissolving excess aluminum halide in a complex consisting of aluminum halide and ethyl acetate (molar ratio = 1:1) (cf. Japanese Pat. Publication No. 3804/1969). However, all of these methods are defective in that, they can produce no more than a polyolefin oil having a relatively high viscosity under the ordinary reaction conditions, and in the case where the reaction temperature is elevated with a view to preparing a low-viscosity polyolefin oil, not only the viscosity index of the resulting polyolefin oil will be reduced but also the yield thereof will decrease.
Another trouble with the conventional methods for polymerization of olefin lies in that the halogenous component of the catalyst would get mixed in the product polyolefin oils. The presence of halogen in the product has a bad influence of grave importance on the process of after-treatment of the polymerization product. For instance, at the time of distilling the monomer and/or dimer from the polymerization product, there takes place the thermal cracking of a part of the halogen in the polyolefin oil giving rise to hydrogen chloride, entailing such a trouble that the distillation apparatus gets corroded thereby. Furthermore, at the time of the hydrogenation of the double bond remaining in the polymerization product for the purpose of improving the heat stability as well as oxidation stability of said product, there occurs such a trouble that the hydrogenation catalyst would be turned to be inactive by the halogen.
Consequently, it is in fact impossible to prepare an olefin oligomer having low viscosity and high viscosity index and scarcely containing halogen mixed therein which is qualified for such uses as gas turbine oil, hydraulic fluids for aircraft, insulating oil, cosmeric base, etc.
Under such circumstances, U.S. Ser. No. 538,965 discloses a method of effecting polymerization of olefin having more than 6 carbon atoms in the presence of a mixture consisting of (a) polyhydric alcohol derivative selected from esters and ethers of polyhydric alcohol and aluminum halide. According to said method, it is possible to obtain an olefin oligomer having a viscosity (in terms of kinematic viscosity at 100.degree. F) of as low as 15 - 45 centistokes and a viscosity index (VIE) of as high as 120-145 and scarcely containing halogen at the yield of about 75-85%.
The present invention aims at further improvement of the method for preparation of olefin oligomer disclosed in said previous invention, and particularly it proposes a method for preparation of olefin oligomer as an improvement of the previous invention by the present inventors from the viewpoints of both the yield of olefin oligomer and the prevention of halogen from getting mixed in the product olefin oligomer.